On Windows, the USB MIDI Inputs and Outputs don’t match 1 to 1. There is an additional MIDI Output in the system, which is why the MIDI Output is offset by 1.
This is unrelated to the Omniport or 5 pin ports.
The list of inputs and outputs are showing the USB virtual MIDI ports from the MC8. There are 4 USB virtual MIDI ports available. In the controller settings, there is a USB Virtual MIDI port setting. If set to 1, MIDI messages are sent only on Port 1 (this would be Input 1 and Output 2 in your case). If set to 2, MIDI messages are sent on Port 1 and 2.
You should try using MIDI Input 1 and MIDI Output 2 first.
Do note that in Windows, if a software is using the MIDI port (like the editor) then it cannot be used by another software.